This invention relates generally to internal combustion engines, and more particularly to turbocharged engines used in locomotives, and specifically to a method and apparatus for cooling a turbocharged diesel locomotive engine.
Internal combustion engines, such as the turbocharged diesel engines utilized for rail locomotives, require cooling systems to limit the temperatures of various engine components. Internal combustion engines are known to be designed with internal cooling passages for the circulation of coolant to remove heat energy from the engine components. Lubricating oil which is circulated throughout the engine to reduce friction will also absorb heat and, therefore, will also require cooling to avoid reaching temperatures that would detrimentally affect its lubricity. Diesel engines often utilize turbochargers to increase power by compressing the intake combustion air to a higher density. Such compression results in the heating of the combustion air, which must then be cooled prior to its use to enable the engine to have high volumetric efficiency and low emissions of exhaust pollutants. For mobile applications such as rail locomotives, the only readily available heat sink is the surrounding ambient air. It is known to utilize a pumped cooling medium, such as water, to transport heat to finned radiator tubes. The radiator tubes then transfer the heat to the ambient air, often using forced convection provided by fans.
It is desirable to maintain an internal combustion engine and its associated intake combustion air at two different temperatures in order to optimize the performance of the engine. U.S. Pat. No. 3,863,612 dated Feb. 4, 1975, and assigned to the assignee of the present invention, describes a cooling system for a turbocharged diesel engine wherein coolant is provided at one temperature to the cylinder jackets of the engine, and coolant at a lower temperature is provided to an intercooler for cooling the compressed combustion air. This system utilizes a single pump, heat exchanger, and temperature control valve to accomplish the dual cooling objectives.
The U.S. Pat. No. 5,415,147 issued on May 16, 1995, also assigned to the assignee of the present invention, teaches a split temperature cooling system for a turbocharged internal combustion engine. This system provides improved cooling capability by utilizing a subcooler in addition to a radiator. The subcooler is located upstream of the radiator in the flow of cooling ambient air. The use of a subcooler provides a greater temperature difference capability between the temperature of the engine and the temperature of the combustion air. Furthermore, this patent teaches a valve system whereby heated coolant may be directed to the intercooler to heat the combustion air during periods of very low ambient temperature when the combustion inlet air temperature would otherwise drop below an optimal value.
The above systems are known as "wet/dry systems" because the coolant is drained from the radiator during periods of low heat rejection demand. An alternative system is taught in U.S. Pat. No. 5,598,705 issued on Feb. 4, 1997, which teaches a "wet system" wherein the water remains in the radiator at all times. The '705 patent teaches a cooling system utilizing two separate coolant loops. A main coolant loop having a pump and a radiator is used to provide cooling to the engine. An aftercooler coolant loop having a separate pump and radiator is used to provide cooling for the combustion air aftercooler. The utilization of a separate cooling loop allows the aftercooler loop to be sized to bring the engine combustion air temperature as close as practical to the temperature of the ambient air without constraining the size or coolant flow rate of the engine and oil cooler radiator. One of the disadvantages of such a wet system is the possibility of freezing of the water in the system, particularly in the aftercooler coolant loop. To prevent overcooling of the aftercooler or freezing of the water in the aftercooler loop, linking conduits are provided between the two loops to allow heated coolant to flow therebetween.
For locomotive applications, ambient air flow through the radiators is normally provided by a multi-speed fan, since the radiators are positioned on the roof of the locomotive. It is known that cycling of a fan between speeds causes excessive heating of the fan motor, excessive power usage, and possible premature failure of the motor. As the demand for more efficient locomotive engine operation increases, the range of allowable temperatures for optimal operation have become narrowed. Prior art cooling systems are thus more prone to excessive fan cycling in an effort to maintain temperatures within desirable narrow ranges. Furthermore the United States Environmental Protection Agency has announced more restrictive emissions limits for oxides of nitrogen to be effective on Jan. 1, 2002. One approach for meeting these more restrictive requirements is to restrict the peak temperature of the intake combustion air. The level of combustion air intercooling needed to meet the new EPA NOx requirements will likely not be achievable with prior art cooling systems during periods of peak engine power demand and peak ambient temperature.